The nib necessarily projects from the body of the implement that the user holds while writing. The ink consumed on the writing medium is replaced as it is being used by ink coming from the reservoir and transferred by capillarity into the nib. When the implement is no longer in use, a cap is fitted onto the body of the implement in such a manner as to cover the nib, firstly to protect it from possible impacts and secondly to prevent it from drying out. If the projecting end of the nib remains in the open air, the solvent of the ink which is situated in the end evaporates whilst the pigment of the ink remains therein. That phenomenon can be detrimental in the sense that it may be difficult, or indeed even impossible, to make further use of the implement, even when a significant quantity of ink remains in the reservoir.
To avoid that drawback, manufacturers of writing implements have already proposed a certain number of solutions for increasing the length of time it is possible to leave a nib in the open air without any major difficulty in using the implement again.
A first solution consists in modifying the composition of the ink by adding additives of the film-forming type. During evaporation of the solvent, a film is formed on the surface of the nib which limits further evaporation, said film being very fragile so that it is eliminated when the user applies the nib once again on the writing medium. 
A similar solution is adopted in document J 54019826 in which the nib is coated in a high polymer that is gastight and easy to peel off. By way of example, the high polymer can be a copolymer resin of vinyl chloride and of vinyl acetate, or an EVA copolymer resin. In that document, such a coating is provided merely to prevent the ink from evaporating, and the nib from drying out during storage and transport of the writing implements, given that the high polymer coating is easily removed before use.